Number Pattern

**JadeKiara** · September 19th 2009, 11:02 PM

Consider the pattern.

$5^3=125$

$5^4=625$

$5^5=3125$

$5^6=15625$

Use this pattern to write out the last 3 digits of $5^{15}$ .

How can I work this out?

**CaptainBlack** · September 19th 2009, 11:13 PM

Originally Posted by JadeKiara

Consider the pattern.

$5^3=125$

$5^4=625$

$5^5=3125$

$5^6=15625$

Use this pattern to write out the last digits of $5^{15}$ .

How can I work this out?

1. You do not specify how many digits constitute "the last" digits of $5^{15}$ .

2. A suitable hypothesis might be that the last two digits of $5^n$ for $n\ge 2$ are $25$ .

3. Suppose this true for $k$ that is $5^k=100 \times M+25$ for some integer $M$ , now $5^{k+1}= ...$

NOw can you take it from there?

CB

**JadeKiara** · September 19th 2009, 11:51 PM

Sorry, it was the last three digits. This was easier than I thought! How silly of me!!

Seeing as odd indices always result in ending digits of 125, and even indices result in 625, the answer to this problem would be 125. Am I right?

**CaptainBlack** · September 20th 2009, 01:48 AM

Originally Posted by JadeKiara

Sorry, it was the last three digits. This was easier than I thought! How silly of me!!

Seeing as odd indices always result in ending digits of 125, and even indices result in 625, the answer to this problem would be 125. Am I right?

yes

CB

Thread: Number Pattern

LinkBack

Thread Tools

Display

Number Pattern

Similar Math Help Forum Discussions

Number pattern

Number pattern.

number pattern

Number Pattern sum

Number pattern

Search Tags